Lp 1730726: Fix lp1501781-unaccent_and_squash.pg for PostgreSQL 9.6

PostgreSQL 9.6 apparently exhibits the same behavior as Postgresql 10
when it comes the OE ligature in the unaccent function.  We modify the
test case to take that into account.

Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Ben Shum <ben@evergreener.net>